Back in 2008, a US National Intelligence Estimate (NIE) stated that Iran had abandoned its original plan to acquire nuclear weapons. Although it was widely appreciated, even then, that this NIE was incorrect, and may also have been advanced as a witting falsehood, the document's net effect was to make it exceedingly difficult for Israel to act preemptively against Iran. Accordingly, at the time, Major General (Res.) Aharon Ze’evi Farkash, former Director of Israeli (IDF) Military Intelligence, had warned prophetically: “…the NIE opens the way for Iran to achieve its military nuclear ambitions, without any interference.”

Now, more than five years later, in October 2013, General Farkash's view has been proven correct. There has, of course, been no defensive first-strike against a steadily nuclearizing Iran, and, in consequence, the Islamic Republic is now likely, within a year or two, of being able to deploy operationally usable nuclear weapons. Naturally, if (when?) that happens, the preemption option will have been lost altogether.

At that conspicuously immobilizing point, the expected costs of any preemption would almost certainly exceed the expected gains. In other words, at that already too-late date, any permissible expression of "anticipatory self-defense" will have become irrational.

This is not, however, a jurisprudential issue. International law is not a suicide pact. Israel has never been under a legal obligation to sit back passively, and quietly await annihilation from a country that remains determined to destroy it.

This is, nonetheless, an operational issue. In this connection, lawfulness and operational readiness are two entirely distinct and discrete considerations. From a purely operational standpoint, the US Air Force (USAF) remains better prepared to launch any eleventh-hour defensive first strikes against Iranian hard targets, than does the substantially-smaller Israel Air Force (IAF).

In principle, at least, any residual U.S. expression of anticipatory self-defense against Iran could have multiple beneficiaries. It could do more than protect Israel from any prospective Iranian nuclear aggression, although any such protection would clearly be valuable and proper in its own right. It could also be in the long-term security interests of the United States.

Left unimpaired, Iranian nuclear assets and materials could sometime be shared with certain Iranian surrogate or otherwise aligned groups, including Hezbollah, or assorted Jihadi organizations. These groups could then uniquely threaten the American homeland. In such fearful matters, missiles are not the only possible nuclear delivery vehicles about which we would need to worry. Armed with "higher-order" forms of ordnance, anti-American terrorists could do just fine with cars, trucks, or ships.

With regard to this still-unobstructed Iranian nuclear menace, American and Israeli interests remain essentially congruent. But if neither Washington nor Jerusalem should display an apt willingness to strike preemptively against a potentially genocidal regime in Tehran - now perhaps, at least in part, because of a newly-opened Barack Obama rapprochement with Iranian President Hassan Rouhani - Israel may have no reasonable alternative to “living with" a nuclear Iran. In such complex and problematic circumstances, the task in Jerusalem would be to forge a durable system of regional nuclear deterrence, one that could function reliably in the long-term, even if there should sometime occur an Iranian lapse from rationality in strategic decision-making.

Israel has a very capable and sophisticated program for ballistic missile defense (BMD), most visibly, the Arrow or Hetz. Yet, defending “soft” targets (civilian populations) from nuclear attack would require a near 100% reliability of interception, and no system of ballistic missile defense could ever be dependably “leak proof.” Moreover, as already noted, even if Israel’s Arrow could intercept all incoming enemy missiles, Iran might still plan to share some of its developing nuclear weapons and technologies with Hezbollah proxies, or with other threatening insurgents

In theory, at least, should the U.S. decide to launch preemptive strikes against pertinent Iranian hard targets, its strike force could comprise approximately 75 stealth attack aircraft – B2s and F22s. Also included would be some 250 nonstealth F15s, F16s, B52s, and three carrier battle groups. Each carrier battle group could contain over 120 F18s, and a large inventory of cruise missiles.

The U.S. has over 1000 UAVs for intelligence, surveillance, and reconnaissance in the region. These could support a decisive campaign to convince Tehran that we could hit Iranian nuclear development facilities, command and control structures, integrated air defenses, Air Force and Navy units, and the pertinent missiles, using over 2,500 aim points. If Israel’s BMD systems were maximally efficient in their expected operational reliability, even an irrational Iranian adversary armed with nuclear and/or biological weapons could be controlled.

This means that even if Israel's nuclear deterrent were somehow immobilized by an enemy state willing to risk a massive "counter-value" Israeli reprisal, that aggressor’s first-strike could still be blocked by Arrow. But these needed efficiencies are not achievable.

Significantly, to deal with the still-mounting Iranian nuclear threat, Israeli planners should not simply assume enemy irrationality. Quite the contrary. Although such irrationality could certainly remain a distinct possibility, and would therefore need to be considered carefully, it is by no means the only cause for strategic concern in Jerusalem and Tel-Aviv, and in Washington.

Even a fully rational nuclear Iran could represent a significant existential threat. This is because a rational adversary could still be subject to misinformation, miscalculation, unauthorized firings, cyber-distortions, and various corollary forms of electronic, mechanical, or computer malfunction.

At this point, however, let us be straightforward. Any American willingness to preempt against Iran will already have effectively disappeared. If only from the political standpoint, it is essentially inconceivable that any American president would still resort to any such employment of devastating U.S. military force.

Israel, it follows, must take certain immediate and far-reaching steps to strengthen and sustain its national nuclear deterrence posture.

To be deterred, any rational adversary, including Iran, would always need to calculate, inter alia, that Israel's second-strike forces were sufficiently invulnerable to first-strike attacks. Facing the Arrow, etc., this adversary would now require incrementally more missiles to achieve an assuredly destructive first-strike against Israel. The Arrow could thus help compel a rational enemy to delay any intended first-strike attack against Israel, at least until such time as it had calculated a capacity to deploy more substantial offensive missile forces.

Hence, the Arrow, even if unsuited for meaningful protection of civilian populations, could support Israeli deterrence by "buying time.

Israel must continue to develop, test, and implement an active defense interception capability to best match the growing threat dictated by Iranian nuclearization and missile capabilities. It must also take appropriate steps to assure relevant decision-makers in Tehran that Israel's nuclear weapons are usable, survivable, and penetration-capable (able to penetrate Iran's own active defenses). Jerusalem, therefore, will soon have to consider, among other things, taking Israel's bomb out of the "basement."

In less metaphoric terms, Israel will need to plan to shift explicitly from its longstanding policy of deliberate nuclear ambiguity, and toward a decisively new policy of selective and partial nuclear disclosure. While this fundamental shift would not be celebrated in Washington, or in any other capital, for that matter, it would still be in Israel's indisputably rational obligation to maintain a credible national nuclear deterrence,

To be sure, Iran should never have been allowed to proceed, without meaningful interference, toward nuclear weapons. Now, however, in the likely absence of any eleventh-hour resort to anticipatory self-defense, Israel will need to take assorted prudent steps to ensure that its indispensable nuclear deterrent is aptly reliable, recognizable, and robust.

LOUIS RENÉ BERES (Ph.D., Princeton, 1971) is Professor of Political Science and International Law at Purdue University. He is the author of many major books, monographs, and articles dealing with nuclear strategy and nuclear war, and of several principal Working Papers presented at the annual security conference in Herzliya, Israel. Professor Beres was Chair of Project Daniel (Israel, PM Sharon, 2003).

THOMAS MCINERNEY ( Lt. General, USAF (Ret.), is a prominent military strategist, and a frequent guest on nationally syndicated talk shows. Co-author of The Endgame, Winning the War on Terror (with Major-General Paul E. Vallely, USA/Ret.), General McInerney is a retired assistant vice chief of staff of the U.S. Air Force, and a Fox News Military Analyst.